1. Field of the Invention
The present invention relates generally to telephone communication systems, and more particularly to a cordless telephone system employing a full duplex FM radio frequency link between a base station which can be connected directly to a user's telephone lines and a remote cordless handset which is portable to permit an individual to carry the unit so that the telephone system may be used at points distant from the base unit.
2. Description of the Prior Art
Since the invention of the telephone personal communications between individuals has grown so that the majority of families in the United States now have telephone service. Rather than reaching a saturation point where each family is equipped with a single telephone, the trend has continued to the point where many families are now supplied with not one but several telephones for use in their homes.
The use of the second, third, etc. telephones as extension phones throughout the home has become increasingly popular. Previously, when only one telephone was installed in a home, a central location for this instrument was chosen. However, no single location can be perfect, and as a result, a person in the home is often forced to come some distance, particularly in a large home, to the location of the telephone either to make or to receive a telephone call.
Likewise, if the inhabitants of the home were to be in the yard, porch, basement, etc., the lack of proximity to a telephone instrument would result in extra steps and the possibility of missing a telephone call if the person were unable to respond by removing the handset from the telephone instrument within a reasonably short period of time after the ringing signal announced an incoming call.
Several solutions to these problems have been proposed. Among these are the use of many telephone extensions placed in convenient locations throughout the area. The other is the use of a multiplicity of telephone line jacks allowing a subscriber to move the telephone from one location to another to always be within easy access. Still another is the use of long extension cords to connect the telephone instrument to its terminal while allowing limited freedom in relocating the telephone.
All of these solutions lack any real flexibility since the location of the extension telephone or the extension jacks may be useful at times, but on many occasions, will be inadequate. In this so-called trend toward outdoor living today, various members of the family are frequently required to go from the garden or the patio to the nearest phone to answer it. In many cases this may be considerable distance and important telephone calls can be missed.
The most satisfactory solution to the extension telephone problem is the use of a telephone extension instrument that requires no wire connections between the instrument and the telephone line. Such an instrument commonly utilizes a radio link between terminal equipment connected to the telephone line and a remote instrument which allows the subscriber to take the instrument any place with a specified range of the telephone line terminal equipment located in his home or the like and permit him to both send and receive telephone calls from any given location. Such a wireless extension telephone could be taken to the garden, patio, basement, upstairs, downstairs, placed in any room of the house where convenient, as well as possibly even carried by the subscriber while visiting neighbors, etc., so that in-coming calls would not be missed and so that they might be rapidly answered without substantial movement on the part of the subscriber.
The advantages that such wireless telephones offer to subscribers are readily apparent. However, it should be pointed out that this same type of instrument will find considerable use in restaurants and other commercial establishments where the person to whom the in-coming call is directed can have the telephone brought to him for answering without the necessity for intervening extension cords or other cumbersome extensions. The number of possible uses for a wireless extension telephone in business, industry, etc., are limited only by the imagination of the user.
In order to overcome these problems, the prior art has evolved a number of types of cordless telephone systems. A typical cordless telephone system includes a cordless telephone handset and a base unit. The base unit is connected to the telephone lines and includes an antenna, a transmitter and a receiver for communicating an RF carrier signal that is modulated by telephone switching signals and audio signals to the cordless telephone. The cordless telephone handset includes a speaker and a microphone and also an antenna, transmitter and receiver for likewise communicating telephone switching signals and audio signals to the base unit via an RF carrier signal. The RF carrier signal transmitted by the cordless telephone usually has a different center frequency from the RF carrier signal transmitted by the base unit.
Switching signals representative of numeric characters may be generated in the cordless telephone by operating a typical telephone mechanical dialing mechanism to generate dialing pulse signals. Switching signals for initiating "off-the-hook" and "on-the-hook" switching functions may be generated by removing the telephone handset from its cradle and by replacing the same respectively. In a typical prior art cordless telephone the removal of the handset enables the transmitter and receiver in the cordless telephone for communication signals to the base unit.
Upon enablement of the transmitter, a control signal is generated in the cordless telephone and transmitted to the base unit. Upon receipt and detection of the control signal the base unit seizes the telephone lines so as to enable audio signals and switching signals from the cordless telephone that are received by the base unit within a given audio frequency band to pass onto the telephone lines.
Cordless telephone systems of the prior art are typically battery powered and include a switch for energizing or de-energizing the telephone so that the battery can be preserved while it is not desired to use the cordless telephone. When the cordless telephone is energized, it is enabled to receive and detect a "ring" signal from the base unit and to feed the ring signal to a speaker.
While the above-described cordless telephone is typical of the prior art, many different United States patents have been issued relating to cordless telephone systems and specific improvements therein. Examples of prior art United States patents relating to cordless telephones include U.S. Pat. No. Re. 26,459 which issued Sept. 17, 1968 to J. H. Lemelson; U.S. Pat. No. 3,193,623 which issued on July 6, 1965 to R. V. Burns et al; U.S. Pat. No. 3,254,159 which issued on May 13, 1966 to E. R. Condict; U.S. Pat. No. 3,286,184 which issued on Nov. 15, 1966 to F. H. Blitchington, Jr.; U.S. Pat. No. 3,366,744 which issued on Jan. 30, 1968 to M. Miller; U.S. Pat. No. 3,366,745 which issued on Jan. 30, 1968 to M. Miller; U.S. Pat. No. 3,366,880 which issued on Jan. 30, 1968 to J. P. Driver; U.S. Pat. No. 3,443,035 which issued on May 6, 1969 to T. E. McCay; U.S. Pat. No. 3,449,750 which issued on June 10, 1969 to G. H. Sweigert; U.S. Pat. No. 3,476,882 which issued on Nov. 4, 1969 to J. H. Vogelman et al; U.S. Pat. No. 3,535,689 which issued on Oct. 20, 1970 to H. S. Oden; U.S. Pat. No. 3,557,312 which issued on Jan. 19, 1971 to J. H. Bogelman et al; U.S. Pat. No. 3,586,787 which issued on June 22, 1971 to J. H. Vogelman, et al; U.S. Pat. No. 3,644,681 which issued on Feb. 22, 1972 to D. E. Rice; U.S. Pat. No. 3,840,811 which issued Oct. 8, 1974 to R. D. Blouch; U.S. Pat. No. 3,919,491 which issued on Nov. 11, 1975 to C. L. Luce; U.S. Pat. No. 4,032,723 which issued on June 28, 1977 to L. E. Mendoza; U.S. Pat. No. 4,039,760 which issued on Aug. 2, 1977 to L. D. Gregory et al; and U.S. Pat. No. 4,053,717 which issued on Oct. 11, 1977 to D. E. Snider. These patents are illustrative of the state of the art and represent many different approaches at solving the various problems relating to cordless telephone systems.
While the prior art attempted to solve the same general problems, many different approaches were taken and many new and different problems arose. Most of the systems require that the remote handset be operated within relatively short distances from the base unit and many experience interference from various energy sources in the area, cross-talk between transmitted and received signals, excessive noise, and the like while most systems claim to have great range, most have an effective range of about 200 feet. Similarly, line seizure was often inadvertently terminated in systems where maintaining line acquisition requires the continuous receipt of a generated tone. The range of the system, noise levels, interference, etc. are to a large part related to the particular frequencies used, and this is to some extent governed by FCC regulations.
Another continuing problem exists where an unauthorized user in possession of a cordless remote handset utilizes the handset to place unauthorized calls over a subscriber's telephone lines with the result that the subscriber is billed for the unauthorized calls.
Many of the prior art patents listed above attempted to solve one or more of these problems but none successfully solved even a small portion thereof. In other areas, such as radio telephones, conventional radio transmitters and receivers, and conventional telephone systems, inventors have begun developing sophisticated improvements to such systems employing microprocessor controls, dialing memories, security features and the like. Examples of such prior art patents include U.S. Pat. No. 3,590,166 which issued on June 29, 1971 to A. Anschutz; U.S. Pat. No. 3,931,565 which issued on Jan. 6, 1976 to S. Amoroso; U.S. Pat. No. 3,961,261 l which issued on June 1, 1976 to P. W. Pflasterer; U.S. Pat. No. 3,962,644 which issued on June 8, 1976 to W. Baker; U.S. Pat. No. 4,000,468 which issued on Dec. 28, 1976 to J. R. Brown; U.S. Pat. No. 4,008,380 which issued Feb. 15, 1977 to R. C. LaBorde; U.S. Pat. No. 4,059,728 which issued on Nov. 22, 1977 to C. P. Milkes; U.S. Pat. No. 4,087,638 which issued May 2, 1978 to J. C. Hayes et al; and U.S. Pat. No. 4,122,304 which issued Oct. 24, 1978 to T. O. Mallien.
In spite of all of these advances a low cost, reliable and compact cordless telephone system which is compatable with all existing telephone systems and which meets all state and federal regulations had still not been achieved.
Thus, a relatively simple, inexpensive, trouble-free cordless telephone system having substantially greater range, reliability and security was required. The present invention provides such a cordless telephone system and employs several unique features which increase the range and reliability of the system while reducing noise and undesirable cross-talk; which provides some measure of privacy while simultaneously making it difficult for an unauthorized user to place calls on a subscriber's system; which reduce the size while increasing the portability of the cordless handset thereby solving substantially all of the problems of prior art cordless telephone systems.